There is an ever increasing demand for easily processed thermoplastic resins which are thermally stable at higher and higher temperatures. Recent developments have produced many heavily aromatic polymers typically polyimides, having the desired thermostability, but lacking the desired processibility.
The difficulty of processing prior art polyimides is exemplified by the roundabout techniques which have been developed. For example, pyromellitimide is typically processed to the desired physical shape as a polyamic acid, an intermediate chemical form, and then heated to form the more stable imide linkage. Several other techniques specifically designed to avoid thermoforming polyimide resins are also being used.
One key in developing a melt processible material is maintaining adequate flexibility along the polymer backbone. In the subject polyimides, an alkylene linkage between the imide groups is provided which is believed to provide this flexibility. In general, polyimide resins, which are typically the condensation polymerization reaction product of a dianhydride and a dianiline, are likely candidates in the development of thermally stable resins, and limited success has been achieved in this area prior to this invention. For example, as reported in U.S. Pat. No. 3,832,322, aromatic fluoro-alkylene-linked polyimides compounds have been synthesized. While these compounds may have many of the desired properties, their cost is relatively high.
The efforts which produced the subject invention were directed toward the general goal of synthesizing novel polyimide resins which would be serviceable in high temperature environments and easily processed. Therefore, it is an object of this invention to provide aromatic alkylene-linked polyimide which are thermally stable up to temperatures of at least about 400.degree. C. and processible in conventional thermoplastic equipment. In addition, a fire resistant or at least a slow burning resin was desired.
It is a broader object of this invention to provide an alkylene-linked thermoplastic polyimide resin wherein the repeating mer unit has the following structural formula: ##STR1## wherein n may vary from 3 to 18 and R is a divalent organic radical selected from the class consisting of (a) aromatic hydrocarbon radicals having from about 6 to about 20 carbon atoms and halogenated derivatives thereof, (b) alkylene radicals having from about 1 to 20 carbon atoms, (c) oxyalkylene radicals having from about 1 to about 100 carbon atoms, and (d) divalent hydrocarbon radicals having the following formula: ##STR2## wherein R.sub.1 is selected from the group consisting of ##STR3## or an alkylene group having up to 15 carbon atoms.
In the following description of the subject invention, frequent reference will be made to the attached sheet of structural formulae and reactions in which:
Formula I depicts alkylene-linked dixylyl compounds suitable for use in the preparations of the subject polyimides.
Formula II depicts suitable alkylene-linked aromatic dianhydride compounds for use in the preparations of the subject polyimides.
Formula III depicts the preferred repeating unit in the subject alkylene-linked aromatic polyimides.
Reaction A depicts the Friedel-Crafts type reaction which produces the subject dixylyl compound shown in Formula I.
Reaction B depicts the oxidation reaction which transforms the dixylyl compound of Formula I into the corresponding tetracarboxylic acid.
Reaction C depicts the dehydration of the tetracarboxylic acid produced in Reaction B to form the alkylene-linked dianhydride compound of Formula II, and
Reaction D depicts the subject condensation type polymerization reaction in which the subject dianhydride is reacted with a dianiline compound to form the preferred aromatic alkylene-linked polyimide.